A large amount of C5 cuts is produced as by-products in the process for producing ethylene by petroleum cracking. The C5 cuts are rich in dienes such as isoprene, cyclopentadiene and 1,3-pentadiene. The dienes are reactive and thus are important raw materials in chemical industry. The amount of dienes in C5 cuts may vary depending upon the cracking raw materials, cracking degree and separation degree of the petroleum hydrocarbons, but the amount of total dienes in C5 cuts is in the range of from 40% to 60%. Therefore, it is of great significance to separate and utilize C5 cuts in order to improve the economic efficiency of ethylene units and fully utilize petroleum resources.
The C5 cuts from a cracking process consists of more than 20 components which have boiling points close to each other and can easily form azeotropes. Thus, a complicated process is needed to separate the valuable C5 dienes from the C5 cuts obtained from a cracking process. The C5 cuts contains a small amount of alkynes having 4 or 5 carbon atoms such as butyne-2, pentyne-1, pentyne-2 and isopropenyl acetylene (hereinafter referred to as valylene). The amount of the total alkynes is about 0.1-2 wt %. Due to their chemical reactivity, the alkynes are major impurities harmful to the reaction and polymerization of C5 diene products. Therefore, it is an important step in the process for separating the C5 cuts to efficiently remove the alkynes.
In order to produce C5 dienes having a high purity, an extractive rectification process is widely used in the art. The advantage of the process is that the amount of the total alkynes in the isoprene product can be reduced to 50 ppm or less while removing a minor amount of cyclopentadiene. The disadvantage of the process is that the energy consumption is high and the alkynes will accumulate in certain units during the separation, thereby resulting in a risk for operation. The conventional selective solvents include dimethylformamide (DMF) and acetonitrile as are used in, for example, JP63101338, U.S. Pat. No. 3,510,405 and U.S. Pat. No. 3,436,438.
U.S. Pat. No. 3,510,405 (Japanese Geon, Co. Ltd., Tokyo, Japan) discloses a process for separating the isoprene having a high purity from C5 cuts, comprising:                subjecting C5 cuts obtained by a hydrocarbon steam cracking or other pyrolysis cracking to a thermal dimerization and separating the resulting dicyclopentadiene;        conducting the first extractive rectification in the presence of a polymerization inhibitor and DMF as a solvent that was recovered in a stripping column for recycling;        rectifying the materials in a heavy fraction removing rectification column to separate heavy fractions comprising cyclopentadiene and 1,3-pentadiene;        conducting the second extractive rectification in the presence of a polymerization inhibitor and DMF as a solvent that is recovered in a stripping column for recycling; and        rectifying the materials in a light fraction removing column and obtaining an isoprene product of high purity at the bottom.        
CN1253130A discloses a reaction rectification process wherein C5 cuts as a raw material is subjected to a reaction rectification without dimerization. The disadvantage of the process is that a large amount of cyclopentadiene is produced at the top of the column and it is difficult to operate the column.
CN1056823A discloses a process for removing most of the alkynes contained in C5 cuts by rectification prior to an extractive separation. An advantage of the process is that the risk of the explosion of the alkynes can be avoided during the course of the separation of C5 cuts due to the removal of most of the alkynes and thus ensures a safe production. Meantime, the load for the subsequent steps and the amount of solvent for the extraction can be reduced and thus the processing ability of the process as a whole can be improved. However, the C5 cuts at the bottom still contain more than 200 ppm of alkynes. Therefore, two-step extractive rectification is necessary to obtain isoprene suitable for polymerization.
Generally, a separation process of C5 cuts comprises two-step extractive rectification and two-step conventional rectification in the prior art, in order to obtain isoprene suitable for polymerization. Furthermore, it is necessary for the solvent to be recovered and refined in certain solvent-recovery unit. Therefore, the energy consumption, investment and operation cost is high due to the complicated procedure. Specifically, there exist following problems: 1) The number of steps is large, and the investment is high. Accordingly, both the operation cost and energy consumption increase. 2) The equipment may be fouled with black dregs and a large amount of tarry materials build up in the solvent-recovering unit, which are difficult to be cleaned; 3) A large amount of the solvent, about 20 kg solvent/ton product is used and thus the production cost is increased.